<!DOCTYPE html>
<html>
<head>
<style>
* {
    line-height: 14px;
}
#a {
    position: relative;
}
#b {
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
}
</style>
</head>
<body>
<div id=a>X
<div id=b>X</div></div>
</body>
</html>

